The present invention relates to a printer carriage bushing suitable for use in connection with a large scale plotter carriage which slides back and forth transversely of the print medium.
U.S. Pat. No. 5,332,321 discloses bushings of this type and discusses the conflicting design requirements imposed thereon. High tolerance bushing systems require dimensionally stable, i.e. rigid materials, such as metals, but these usually have a high coefficient of friction and are relatively noisy in use. However, bushing systems made of lubricious materials, such as plastics, for reduced friction, tend to be dimensionally unstable, especially with relatively heavy carriages and the associated increased forces applied. This can lead to unwanted movement of the carriage on the rod it is mounted on. Small movements have been noticed that affect print quality by mispositioning the drops on the paper. The reason for this is on the one hand the clearances between the carriage and the bushing and on the other hand the elastic deformation of the bushings as well as the part of the carriage in contact with them.
With increasing size of printer/plotter systems, the physical loads imposed upon the bushings by repeated changes of direction at a high speed increase substantially. In addition the problem of noise becomes an important factor.
U.S. Pat. No. 5,346,320 discloses a bushing liner comprising a moulded mixture of carbon, teflon and nylon.
Both U.S. Pat. No. 5,332,321 and U.S. Pat. No. 5,348,404 disclose bushings constructed of sintered bronze.
The present invention seeks to overcome or reduce one or more of the problems mentioned above.
According to a first aspect of the present invention, there is provided a printer/plotter apparatus comprising a carriage mounted for sliding movement on at least one rod, the carriage mounting on the or each rod comprising at least one bushing which is of generally part-annular cross-section and which comprises a relatively rigid material, characterised in that the bushing further comprises one or more portions of a relatively lubricious material, at least one of which forms at least part of the internal surface of the bushing.
According to a second aspect of the present invention, there is provided a bushing liner for a printer carriage and having a generally part-annular cross-section, the bushing liner comprising a relatively rigid material, characterised in that the bushing liner further comprises one or more portions of relatively lubricious material, at least one of which forms at least part of the internal surface of the bushing liner.
Advantages of the above bushing are that the relatively rigid parts provide resistance to the effects of high dynamic forces and high precision, and thus a good print quality, whereas the relatively lubricious parts provide low friction and low noise.
In U.S. Pat. No. 5,332,321 the carriage slider rod is directly engaged by flat surfaces of the bushing. Due to unavoidable tolerances, this arrangement tends to produce high wear when it is first put into use, due to the small area of contact existing at that time. This early wear of the bushing provides a corresponding displacement in the position of the carriage leading to the deterioration in performance mentioned above.
Accordingly the present invention also seeks to provide an arrangement in which a controlled area of the bushing contacts the rod throughout its working life.
According to a third aspect of the present invention, there is provided a bushing for sliding on the carriage slider rod of a printer and having a generally part-annular construction wherein the internal surface of the bushing comprises rib means for contacting the slider rod, characterised in that the rib means comprise one or more circumferentially-spaced sets of ribs extending axially of the rod, the or each set comprising two to four relatively closely circumferentially-spaced ribs. Preferably there are two sets each comprising two ribs.
The use of such a small plurality of ribs provides a satisfactory compromise between the requirements of providing a sufficiently large total contact area to avoid unduly rapid wear and at the same time providing sufficiently accurate contact points capable of adequately transferring forces in well-defined directions.
The radially inner surface of the ribs are preferably accurate, corresponding to the diameter of the slider rod to be contacted.
The ribs may be provided on bushings comprising only a single material.
In U.S. Pat. No. 5,332,321 the two positions of contact subtend a right-angle at the centre of the slider rod. For printer carriages involving a more complicated and variable set of loading forces, it has been determined that a modified arrangement is required.
Thus according to a fourth aspect of the present invention, there is provided a bushing for sliding on the carriage slider rod of a printer and having a generally part-annular construction wherein the internal surface of the bushing defines two circumferentially-spaced positions for contact with the rod, characterised in that the positions subtend an angle of between 100xc2x0 and 165xc2x0.
The angle is preferably within the range 110xc2x0 to 130xc2x0 and is most preferably substantially 115xc2x0.
The bushing may comprise only a single material, and the individual positions of contact may be as disclosed in U.S. Pat. No. 5,332,321.